The present invention relates to apparatus for cleaning and separating materials in general and wood chips in particular.
Paper is typically manufactured from cellulose fibers obtained from wood. The fibers may be separated from the raw wood either mechanically, typically by abrasion, or chemically by dissolving the lignin which binds the fibers in wood together. The process of chemically dissolving the lignin involves reducing wood to chips of uniform thickness which are processed by cooking the chips in a chemical solution until the lignin has been dissolved. To maximize the amount of useful fiber from a given quality of chips the thickness of the chips should be uniform to prevent over thickness chips from requiring excessive digestion time which can result in the degradation of some fiber due to prolonged contact with the digestion liquid.
The process of reducing the raw logs into chips produces a certain amount of dust, fines and pins. Dust or fines are wood particles which are too small to contain useful fiber. Pins are larger particles which contain some useful fiber which can be used in papermaking if the percentage of pulp derived from pins is not too large.
A number of different apparatus have been developed for separating wood chips by size, particularly thickness. These machines include disk screens and bar screens, see for example my earlier U.S. Pat. Nos. 5,476,179 and 5,392,931 to bar screens, and U.S. Pat. No. 4,972,960 to a disk screen. Devices for separating wood chips by ballistic cross-section have also been developed, see for example U.S. Pat. No. 5,409,118. An apparatus known as a chip destructuring device, such as is disclosed in U.S. Pat. No. 4,953,795 cracks large chips to allow chemical liquors to penetrate the chips for more uniform processing. While these devices have been found to be useful and advantageous for separating wood chips by size and removing knots, tramp metal, and rocks, or separating sand and useful wood fines, a device known as a wave screen and described in my earlier U.S. Pat. No. 5,037,537 which is incorporated herein by reference, is particularly useful for separating dust and fines from wood chips. A wave screen has the ability to separate out dust and fines while retaining slightly larger particles, referred to as pins, which contain useful fiber, with the accept flow.
Disk screens, bar screens, and air density separators have also found application in separating and cleaning municipal trash in preparation for recycling some or all components of a stream of trash. A wave screen can also be used to separate or clean certain materials from municipal trash.
A wave screen type machine, while quite useful because of its ability to remove only particles which contain no useful fiber from a stream of wood chips, could benefit from design changes which would facilitate modularity and reduce maintenance, while at the same time increasing utility when used to process municipal trash.